Nitrogen in various chemical combinations is a component of the waste products generated by rearing fish. There are four primary sources of nitrogenous wastes: urea, uric acid, and amino add excreted by fish; organic debris from dead and dying organisms; uneaten feed and feces; and nitrogen gas from the atmosphere. Fish expel various nitrogenous waste products through gill diffusion, gill cation exchange, urine, and feces. The decomposition of these nitrogenous compounds is particularly important in intensive recirculating aquaculture systems (RAS) because of the toxicity of ammonia, nitrite, and to some extent, nitrate. The process of ammonia removal by a biological filter is called nitrification, and consists of the successive oxidation of ammonia to nitrite and finally to nitrate. The inverse process is called denitrification and is an anaerobic process where nitrate is converted to nitrogen gas. The denitrification process is becoming increasingly important as fish stocking densities increase and water exchange rates are reduced, resulting in excessive levels of nitrate in the culture system.
Biological treatment processes employ bacteria that grow either attached to a surface (fixed films) or that grow suspended in the water column. Almost all recirculating systems use fixed-film bioreactors, where the nitrifying bacteria grow on either a wetted or submerged media surface. The ammonia removal capacity of biological filters is largely dependent upon the total surface area available for biological growth of the nitrifying bacteria. For maximum efficiency, the media used must balance a high specific surface area, i.e., surface per unit volume, with appreciable void ratio (pore space) for adequate hydraulic performance of the system. The media used in the biofilters must be inert, non-compressible, and not biologically degradable. Typical media used in aquaculture biofilters are sand, crushed rock or river gravel, or some form of plastic or ceramic material shaped as small beads, or large spheres, rings, or saddles. Biofilters must be carefully designed to avoid oxygen limitation or excessive loading of solids, biochemical oxygen demand, or ammonia.
An ideal biofilter would remove 100% of the inlet ammonia concentration, produce no nitrite, require a relatively small footprint, use inexpensive media, require no water pressure or maintenance to operate, and would not capture solids. Unfortunately, there is no one biofilter type that meets all of these ideals, each biofilter has its own strengths and weaknesses and areas of best application. Large-scale commercial recirculating systems have been moving towards the use of granular filters (expanded beds, fluidized beds and floating bead beds). However, there are many types of biofilters that are commonly used in intensive RAS, such as submerged biofilters, trickling biofilters, rotating biological contactors (RBC), floating bead biofilters, dynamic bead biofilters, and fluidized-bed biofilters.
The submerged biofilter includes a volume of biofilter medium upon which nitrifying bacteria grow. The wastewater flows in either an up-flow or a down-flow direction and thus the hydraulic retention time can be controlled by adjusting the water flow rate. Solids from the culture tank can accumulate within the submerged filter, along with cell mass from nitrifying and heterotrophic bacteria. This process can eventually block the void spaces, requiring some mechanism to flush solids from the filter for successful long-term operation. To provide large void spaces to prevent clogging of the filters, the media used for submerged blotters has been traditionally of large size, such as uniform crushed rock over 5 cm in diameter or plastic media over 2.5 cm in diameter. However, 5 cm diameter crushed rock would only have a specific surface area of 75 m2/m3 and a void fraction of only 40 to 50%. Random packed plastic media would also have a relatively low specific surface area of 100-200 m2/m3, but a much higher void fraction, greater than 95%. Drawbacks of this type of filter include problems of low dissolved oxygen and solids accumulation, resulting from heavy loading of organic matter and the difficulty of backflushing. Although this type of filter was promoted and used in aquaculture in the past, it has since been replaced in aquaculture due to the inherent high construction costs, biofouling problems, and operational expense.
A recent variation on the submerged biofilter, termed a moving-bed biofilm reactor or a dynamic bed biofilter, uses small slightly buoyant polyethylene tubular media (7 mm long and 10 mm in diameter), in a heavily aerated, submerged bed (Rusten, G., et al. Water Environm. Res. 70:1083-1089 (1998)). The tubular media has both internal and external ribs for enhanced surface area and a protected divided interior section to protect the biofilm from being completely stripped off during agitation in the moving bed. The heavy aeration keeps the bed in constant motion, which minimizes dissolved oxygen problems and solids accumulation. These biofilters report low total energy use and a high nitrification rate. The effective surface area for bacterial growth is around 350 m2/m3. One advantage of this type of biofilter is its low hydraulic head and aeration; its disadvantage is the large aeration requirement to maintain the bed in motion.
Trickling biofilters operate in the same way as submerged biofilters, except the wastewater flows downward over the medium and keeps the bacteria wet, but never completely submerged. Since the void spaces are filled with air rather than water, the bacteria never become oxygen-starved. Trickling filters have been widely used in aquaculture, because they are easy to construct and operate, are self-aerating and very effective at removing gaseous carbon dioxide, and have a moderate capital cost. In municipal waste water treatment systems, trickling filters were traditionally constructed of rocks, but today most filters use plastic media, because of its low weight, high specific surface area (100-300 m2/m3) and high void ratio (>90%). A range of trickling filter design criteria has been reported. Typical design values for warm water systems are hydraulic loading rates of 100 to 250 m3/day per m2; media depth of 1-5 m; media specific surface area of 100-300 m2/m3; and TAN removal rates of 0.1 to 0.9 g/m2 per day surface area. Trickling biofilters have not been used in large-scale coldwater systems, probably due to the decrease in nitrification rates that occurs at the lower water temperatures and the relatively low specific surface area of the media. They have found a use in smaller hatchery systems where loads tend to be low and variable.
Rotating biological contactors (RBC) operate by rotating the biofilter media, consisting of disks or tubes, through a tank containing the wastewater. Bacteria attached to the rotating medium are exposed alternately to the wastewater and the atmosphere, which provides oxygen to the biofilm. The medium is typically submerged at a level of 40% of the drum diameter and is rotated at a speed of 1.5-2.0 rpm. Rotating biological contactors have seen some use in fully recirculating systems, because they require little hydraulic head, have low operating costs, provide gas stripping, and can maintain a consistently aerobic treatment environment. In addition, they tend to be more self-cleaning than static trickling filters. The main disadvantages of these systems are the mechanical nature of its operation and the substantial weight gain due to biomass loading of the media and the resultant load on the shaft and bearings. Early efforts using RBC-s often employed under-designed shafts and mechanical components, which resulted in mechanical failure, but a properly designed RBC can be functional and reliable.
The floating bead has become a popular biofilter for the treatment of small or moderate flows, usually less than 1,000-2,000 L/min. The floating bead filters are expandable granular filters that display a bioclarification behaviour similar to sand filters (Malone, R. F. & Beecher, L. E., Aquacult. Eng. 22:57-73 (2000)). They function as a physical filtration device or clarifier by removing solids, while simultaneously encouraging the growth of desirable bacteria. They also remove dissolved wastes from the water through biofiltration. Floating bead filters are resistant to biofouling and generally require little water for backwash. The bead filter is typically either bubble-washed or propeller-washed during its backwashing procedure, which expands the bed and separates trapped solids from the beads. The beads used are food-grade polyethylene with a diameter of 3-5 mm and a specific gravity of 0.91, and a moderate specific surface area of 1150-1475 m2/m3. Bead filters advantages include their modular and compact design, ease of installation, and operation. In addition, they can be used as a hybrid filter for both solids removal and nitrification. Bead filters using propeller-washed backflushing have been built with bead volumes of up to 2.8 m3. Most small-scale systems use the bubble-washed filters, typically less than 0.28 m3.
Fluidized-bed biofilters have been used in several large-scale commercial aquaculture systems (15 m3/min to 150 m3/min or 400 to 4,000 gpm). Their chief advantage is the very high specific surface area of the media, usually graded sand or very small plastic beads. The fluidized-bed biofilter can easily be scaled to large sizes, and are relatively inexpensive to construct per unit treatment capacity. Since the capital cost of the biofilter is roughly proportional to its surface area, fluidized-bed biofilters are very cost competitive and are relatively small in size compared to other types of biofilters (Summerfelt, S. T., in CIGR Handbook Agric. Eng. pp. 309-350 (CIGR, Series Ed., Wheaton, F., Volume Ed.), Am. Soc. Agric. Eng. (1999)). The main disadvantages of fluidized-bed biofilters are the high cost of pumping water through the biofilter and that a fluidized-bed biofilter does not aerate the water, as do trickling towers and RBC-s. Additional disadvantages are that they can be more difficult to operate and can have serious maintenance problems, usually due to poor suspended solids control and biofouling.
In fluidized-beds, water flows through the void spaces in the medium, either upward or downward, depending upon the specific gravity of the medium. The bed becomes fluidized when the velocity of the water through the bed is sufficiently large to suspend the medium in the velocity stream, causing the bed to expand in volume. The resulting turbulent motion of the medium provides excellent transport of dissolved oxygen, ammonia-nitrogen and nitrate-nitrogen to the biofilm and shears off excess biofilm. The result is high nitrification capacity in a relatively compact unit, but at the cost of the high energy required to fluidize the filter medium.
The design of the flow distribution mechanism is absolutely critical for reliable operation of fluidized-bed biofilters (Summerfelt, S. T. & Cleasby, I. L, ASAE Trans. 39:1161-1173 (1996); Summerfelt, S. T., in CIGR Handbook Agric. Eng. pp. 309-350 (CIGR, Series Ed., Wheaton, F., Volume Ed.), Am. Soc. Agric. Eng. (1999)). A variety of mechanisms has been employed to inject the water into the bottom of large fluidized-sand biofilters. Traditionally, some form of pipe manifold, starting at the top of the biofilter and running down through the inside of the reactor, has been used. This header and lateral system creates an additional operating pressure that the pumps must work against, generally on the order of ¼ to ½ of an atmosphere (atm).
The major advantage of fluidized-sand biofilters is their ability to be scaled to capacities to assimilate ammonia production from standing fish biomasses on the order of 50,000 kg. In effect, the fluidized-sand biofilters can be made as large as they need to be to handle a specified fish biomass. Other considerations will dictate the actual fish load, with the primary one being risk associated with catastrophic failure.
All of the above biological filters are designed to perform the same function: oxidizing ammonia and nitrite to nitrate. Thus, the biological filter must be designed to fully oxidize the nitrogen equivalents present in the ammonia produced, with an additional safety margin to account for unforeseen events. From a practical perspective, the biofilter selection is less critical in small production systems, i.e., systems that feed at rates below 50 kg per day, than for larger production systems. In small systems, biofilters can be over-designed and the added cost is generally not of critical importance to the overall economic success of the venture.
Each biofilter described above has advantages and disadvantages that need to be considered during the early design phase. One of the chief advantages of both the trickling biofilter and the RBC is that they both add oxygen to the water flow during normal operation. In addition, they provide some carbon dioxide stripping. In contrast, the submerged biofilters, bead filters, and fluidized-bed biofilters are all net oxygen consumers and rely completely on the oxygen in the influent flow to maintain aerobic conditions for the biofilm. If, for whatever reason, the influent flow is low in dissolved oxygen, anaerobic conditions are generated within the biofilters.
Both the trickling biofilters and the RBC filters have the distinct disadvantage of having low specific surface area medium. Since the capital cost of the filter is proportional to its total surface area, the result is physically large and more costly filters. In contrast, bead filters and especially fluidized-bed filters use media with a high specific surface area. These results in reduced cost and space requirements in comparison to that required to achieve the same surface area in a trickling biofilter or RBC.
An additional disadvantage of the trickling biofilters and the RBC is that they readily biofoul, if suspended solids are not adequately controlled. Carbon-eating heterotrophic bacteria grow 100 times faster than the autotrophic nitrifiers do. Their mass can double in an hour, while it takes nitrifiers days to double. This high growth rate and the associated oxygen demand consequently suffocate the nitrifiers buried deeper in the biofilms, resulting in death and sloughing of the biofilm from the bioreactor surfaces.
Prior to this invention a filtration system has been described, having a chamber with a hydraulic loading area that is divided into a plurality of cells such that each cell has a hydraulic loading area less than 2.3 m2. The system further includes a filter medium, such as microbeads, positioned in each cell to filter water passing through the chamber. Microbeads provide a substrate for bacterial growth during operation of a filtration system. The bacteria growing on the microbeads utilize the ammonia and nitrite as nutrients for even further bacterial growth. The bacterial growth on microbeads also tends to reduce the buoyancy of microbeads.
The limiting factor of the rate of reduction of ammonia in each pass through a biofilter is the rate of diffusion of the reactants through the biofilm. The rate is thus related to the residing time of the water within the medium, e.g. if 50% of ammonia is reduced in a circle, and the residing time is increased by a factor two, 50% of the remaining ammonia will be reduced i.e. a total of 75% of the incoming concentration.
Water channeling is a common problem in filtration systems. If the height of the microbeads in the filtration system is too great, or if the width of the chamber is too great, channeling, i.e. the essentially free flow of water through the center region of the filter, tends to occur. Such channeling decreases the residing time of the reactants through the biofilm, and thus leading to decreased filtering activity. The force that prevents channeling through the medium limits the height as well as diameter of microbead bed. The height of the microbead can typically not exceed 60 centimeters and the area can typically be no greater than about 2,3 m2 for microbeads with strong buoyancy (specific weight 16-30 kg per cubic meter) and diameter between 1-3 mm.